1. Field of the Invention
The present invention relates generally to a recording and reproducing apparatus, and particularly to recording and reproducing apparatus of the digital type, and more particularly to a digital VTR or a digital information recording and reproducing disc apparatus.
2. Description of the Prior Art
Hitherto, several proposals have been made on digital VTRs (Video Tape Records) for recording image signals in digital codes. When the image signal is digitally coded, necessary data rates therefor become a very large value, of typically 100-200 M bits/sec. The key problem at the present time is to achieve a substantially same per-area information recording performance on the recording media as the conventional analog type VTR or disc. As one of measures to solve the above problem a proposal has been made to set a sampling frequency at a frequency lower than the Nyquist rate (double the necessary highest frequency of the image signal), thereby to decrease the data rate. This method is called a sub-Nyquist sampling method. The fact that there is a need to set the sampling frequency to be greater than twice the frequency of the above-mentioned necessary highest frequency is well known as sampling and is referred to as the Nyquist rate. It is also known that when using correlation in the image signal, the image signal can be reproduced substantially without any deterioration of its quality using a so-called sub-Nyquist sampling method. The sub-Nyquist sampling method is further classified into two kinds: A first one uses intra-field correlation, and a second one uses inter-field correlation. In view of the picture quality, the latter, i.e., the sub-Nyquist sampling method using the inter-field correlation is superior to the former one, which uses the intra-field correlation. The present invention relates to an improvement in the inter-field sub-Nyquist sampling method.
By applying the inter-field sub-Nyquist sampling method to a digital VTR or digital disc recorder, a recording and reproducing apparatus of good picture quality and low recording medium consumption is obtainable. Since VTRs and the recording disc apparatus use substantially the same principle, hereinafter the description is made taking the VTR as example, but the same applies also to the disc type recording apparatus.
Let us define that reproducing mode of recorded information with a tape running at the same speed as the recording is normal reproducing mode, then a VTR has a need that it must reproduce an image signal recorded on a tape which is driven at such a high speed as 10 times the speed of the normal reproducing mode with sufficient quality of picture to be produced. The digital VTR using the inter-field sub-Nyquist sampling method has a necessity to attenuate an alias component of the reproduced signal by passing through an interpolating filter, wherein correlation between two or more neighboring fields is utilized in the reproduction of the signal. In the normal reproduction mode, the interpolation can be made by delaying the correlation signal of the previous field for a period of one field. However, in the high speed reproduction mode, due to the necessity of skipping reproduction, taking of correlation signal of the previous field is impossible. Furthermore, in a slow reproducing mode wherein the reproducing is made in a slower speed than the normal reproducing mode, sometimes the taking of correlation signal of the previous field may be impossible. Still furthermore, in a still-reproducing mode, wherein a signal for one field only is reproduced, it is impossible to take the correlation signal with the previous one field, too. Thus, in the digital image information recording apparatus using the inter-field sub-Nyquist sampling method, there is a shortcoming that in special picture reproducing modes such as high speed reproducing mode, slow speed reproducing mode and still reproducing mode, the picture quality is very bad.